One example of a thermal print system is disclosed in U.S. Pat. No. 4,250,511, which uses a thermal transfer ribbon having arranged in a repeating series of stripes the three basic colors yellow, magenta, and cyan as well as black. The stripes are disposed perpendicular to the ribbon's direction of transport and they span the whole length of the print line, i.e., the whole print media width. The heat applying print head is formed by a strip of thermal elements arranged in a row transverse to the print media and ribbon transport direction. Each element is connected to a ground lead and to a selection lead. Control means selectively energizes the required leads for desired color transfer. The print media is pressed against the colored surface of the thermal ribbon by a page wide roller whose axis is parallel to the print line. The thermal ribbon itself is kept against and supported by the stationary arranged print head so that the print line is formed by the nip between the print head and the backing roller. Upon printing any one of the thermal elements may be energized to transfer a spot of a particular color of the color stripe which is at that instant over the head. To permit the deposit of any color at a given location on the print media, the ribbon is advanced at a faster rate than the print media.
For generating prints of higher quality more picture elements (pel) per given distance are necessary. The higher rate of pels (e.g., 100 pel per inch) requires smaller thermal elements, e.g., those covering an area of, for instance, 10 mils (254 micrometer). This obviously requires more leads per given area which makes the head much more difficult to manufacture.
In addition to the above, the formulation of the thermal transfer ribbon material is of great importance. That is there are basically two kinds of thermal ribbon: one ribbon uses wax as the transfer media and the other uses resin. With a wax type of ribbon less thermal energy is necessary for softening the material for transferring it to the print media, since wax has a lower softening temperature. Additionally, the time needed for cooling and drying on the print media is longer and the material tends to run. On the other hand, with a resin type of ribbon a higher temperature is necessary for melting the color material for transferring it to the right media, but the time required for cooling and hardening on the print media is shorter and the material does not run or smear. Consequently, for the higher resolution and faster printing a resin type ribbon is best suited.
With a page wide heat applying print head having one or two continuous rows of heating elements transverse to the print media transport direction, as shown in FIGS. 3 and 5 of the above-cited U.S. patent, problems arise in connection with condensing the heating elements. For higher print quality a higher pel number is required and thereafter, at the same time the number of heating elements and selecting leads would have to be increased. Due to considerations of dot to dot spacing and to minimum allowable lead widths the known continuous line area head must allow access to both sides of the array of heating elements and/or is restricted in its resolution by these considerations. Furthermore, only one or two lines of dots, much less than the height of a normal character, can be printed at a time.